I swear I responded to this yesterday. I must have closed my browser before pressing send.
In any case, there is no built in support for adding custom buttons directly into the toolbar. However you can create a custom button and either float if above the map next to the toolbar (position absolute...), or use the custom control framework for the map.
Here is a simple example that adds buttons above the map: https://azuremapscodesamples.azurewebsites.net/?sample=Custom%20navigation%20controls
There are many examples of custom controls for Azure Maps here: https://github.com/microsoft/Maps/blob/master/AzureMaps.md